1. Field of the Invention
This patent specification relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming capable of effectively performing a color image position adjustment.
2. Description of Related Art
Conventionally, color image forming apparatus that form a color image using a number of different color toners often cause a defective phenomenon that images of different color toners are displaced relative to each other. This typically causes a blurred color image. Therefore, these color image forming apparatus are required to adjust positions of color images to precisely form a single color image with an appropriate color reproduction.
Japanese Patent No. 2573855, for example, describes an exemplary color position adjustment and a test pattern used in the color position adjustment. Also, several other test patterns are described in published Japanese unexamined patent applications No. 11-65208, No. 11-102098, No. 11-249380, and No. 2000-112205. In the image forming apparatus disclosed in these documents, a plurality of photosensitive drums form a predetermined test image pattern using a plurality of color toners on both longitudinal sides of an image carrying surface of an image carrying member. The predetermined test pattern is detected by a pair of optical sensors. Based on this detection, displacements of the color images relative to each other are calculated and are used to justify the positions of the color images. More specifically, the predetermined test pattern includes a plurality of marks and the reading of the marks allows an analysis of a displacement of each color from a predetermined reference position. For example, the color position adjustment calculates a displacement dy in a sub-scanning direction y, a displacement dx in a main scanning direction x, a displacement dLx of an effective line length in a main scanning line, and a skew dSq in the main scanning line.
Particularly, the above-mentioned Japanese patent No. 2573855 describes an image forming apparatus capable of moving a reflective mirror arranged on a light path with a stepping motor to adjust a magnification, a slant in the sub-scanning direction, and a parallel movement so as to correct a registration. Also, this image forming apparatus is capable of controlling a drive of a photosensitive drum or a transfer belt to correct a registration.
However, the above-mentioned color position adjustment is not automatically performed by the image forming apparatus. The present inventors have recognized that at present there is no such image forming apparatus that can automatically perform a color position adjustment operation.